Remove references to the backup jobs exemption flag.

The changes associated with this flag were never launched and there are
no plans to launch this in the near future. Remove the references to
this flag.

Bug: 382295619
Test: builds
Flag: EXEMPT removing android.app.job.backup_jobs_exemption
Change-Id: Ia08cee9de9483d4d6018a491463ab953e97bf9b6